The use of latches to retain doors closed but not locked is widespread, particularly for internal doors of residences or offices where locking security is not required. Latches have a lock bolt that is spring-biased to a latching position, in which the lock bolt protrudes from an edge face of the door. Turning a knob or lever associated with the latch retracts the lock bolt to its unlatching position in which the lock bolt is retracted into the door, to allow the door to be opened. The knob or lever is also spring-biased to a position corresponding to the latching position of the lock bolt. This is typically horizontal for a lever handle.
When the door is closed without turning a knob or lever, the lock bolt initially engages a strike plate attached to a door jamb which urges the lock bolt to retract into the door, into the unlatching position. The lock bolt then travels across the strike plate and enters a recess in the strike plate. This allows the lock bolt to return to its (protruding) latching position to retain the door closed.
A known simple latch uses a single spring to bias the latch mechanism and the knob or lever to the latching position. A spring of sufficiently high stiffness to return the knob or lever to the latching position often makes it difficult to close the door without also turning the knob or lever, which is inconvenient. A spring of sufficiently low stiffness to allow easy door closure without turning of the knob or lever often does not return the latch mechanism and the knob or lever to the latching position (which can result in lever sag) and so won't reliably latch the door.
Another known latch uses a single spring for biasing the lock bolt and a separate return mechanism in the door furniture to return the knob or lever to the latching position. This complicates the production and assembly of the latch.
Another known latch utilises a relatively stronger spring to return the knob or lever to prevent sag and a relatively weaker spring to bias the lock bolt to provide easy door closure. A disadvantage of known latches of this type is that they require the knob or lever to be rotated through at least 75° to sufficiently retract the bolt for door opening.
Other known latches require that components of the latch mechanism protrude significantly outside of the latch casing. A disadvantage of known latches of this type is that they requires a large cross bore hole in the door to accommodate the protruding components and equally large door furniture to conceal the bore hole.
It is the object of the present invention to substantially overcome or at least ameliorate one or more of the above prior art disadvantages.